1. Field of the Invention
This invention relates to cast vehicle wheels and particularly, but not exclusively, to vehicle wheels of diecast light alloy intended for use in heavy-goods and large passenger vehicles.
2. Summary of the Prior Art
High loads and high speeds have of recent times resulted in the necessity for the provision of truck, bus and coach wheels of ever-increasing strength in order to resist over long periods of use, a very large number of stress reversals in certain parts of the wheel structure and in order to achieve this, steel wheels of ever greater weight have become necessary.
This increase in weight gives rise to a number of disadvantages including increased tare weight of the vehicle, more difficult handling under maintenance conditions and higher inertia under running conditions. The latter effect is particularly undesirable on steerable wheels since the effort required to overcome the gyroscopic effect is directly related to the weight of those wheels and a significant amount of power is absorbed by power-steering systems which is disadvantageous, particularly if the engine is operating at lower speeds during negotiation of a curve.
It has already been proposed to meet these difficulties by the provision of wheels of cast light alloy such as aluminum, but although weight saving is achieved with such wheels this is not as great as is desirable since light alloys are, like other metals, liable to fatigue fracture as a result of stress reversals and the thickness of the wheel has to be greater than is desirable.
Furthermore, in order to avoid over-heating of braking systems, it is necessary that air should be allowed to circulate freely or even be forced over and around the corresponding brake drum or brake disc and this necessitates the provision of apertures of substantial size in the disc portion of a wheel outwardly of the nave. The apertures should be as large as possible to provide for good airflow, but should be as small as possible to avoid weakening of the disc structure. It has been found that even if the apertures are to provide no more than adequate air cooling, the material remaining between the apertures, which can be equated to spokes, is insufficient to prevent fatigue cracks developing. Quite clearly such a possibility cannot be allowed.